[R] changes to terms.formula in 2.3.x

[Sundar Dorai-Raj]

Hi, all,

I just recently noticed a change in terms.formula from 2.2.1 to 2.3.1 
(possibly 2.3.0, but I didn't check). Here's the problem:

## 2.2.1
 >  update(~ x | y, z ~ .)
z ~ x | y

## 2.3.1
 > update(~ x | y, z ~ .)
z ~ (x | y)

and in the NEWS for 2.3.1

     o	terms.formula needed to add parentheses to formulae with
	terms containing '|'.  (PR#8462)

So, there must be a reason for this change. However, in the lattice 
framework I have this causes many problems because now the `|' is 
interpreted as a logical OR. Could someone suggest a workaround for me 
so that I still get the 2.2.1 behavior my code relies on. Here's a very 
simple test function (works in 2.2.1, fails in 2.3.1):

library(lattice)
foo <- function(x, data) {
   x <- update(x, y ~ .)
   xyplot(x, data)
}
z <- expand.grid(x = 1:10, g = letters[1:3])
z$y <- rnorm(nrow(z))
foo(~ x | g, z)

Thanks,

--sundar